This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Members of the conserved MAP65 protein family play a key role in formation of several microtubule based structures such as assembling the central spindle during cytokinesis in animals and interphase microtubule arrays in plants and fungi. These proteins are thought to stabilize specific microtubule structures by preferentially crosslinking microtubules in an anti-parallel orientation. By sequence, proteins in this family exhibit a modular domain organization comprising of a dimerization domain, microtubule binding region and a regulatory domain. However, we lack a clear structural basis of how these proteins bind microtubules and how orientation-specific crosslinks are achieved. To address this, we are carrying out crystallographic studies on human PRC1, a protein in the Map65 family, required for the successful cell division.